On Wednesday, both the EU and Iran confirmed the talks would resume on Thursday. Iran’s deputy foreign minister Abbas Araqchi explained that Tehran was prepared to continue the talks after the EU "made the assurance that the world powers, particularly the United States, will continue the talks in goodwill and that they are serious about implementing the deal."

Diplomats said much progress had been achieved in the four-day meeting on December 9-12 in Vienna, and expressed hope they could wrap up the practical discussions at meetings in Geneva on Thursday and Friday.

“We were at an advanced stage in Vienna,” a diplomat from one of the six world powers told Reuters. “A lot of work has been done so we can go very fast.”

The diplomat, who spoke on condition of anonymity, cautioned, however, that some western diplomats were concerned Iran could be “more difficult” in the technical discussions because of Washington's decision to expand sanctions this month.